It is important I think that we are never specifically told what happened to trigger off the Ruin that destroyed our version of humanity and plunged humans into a kind of dark age where they lost all their technological sophistication and knowledge. We are given a few tantalising clues, such as in the beginning of Chapter Three, where we are told about the Ruin Song:

Most especially it was frightening when it recalled the Ruin, the end of the civilisation of the ancestors. Verses told of smoky, poisonous fumes, of great fractures in the earth itself, of the way huge buildings toppled and were swept away by the seas.

Later, reference is made to skyscrapers exploding in flames. I think it is significant that we see humanity long after the Ruin, and this is enforced by the way that no-one can remember what precisely happened. However, if we think of what we are told in the above quote, the reference to huge buildings being knocked down by the sea indicates that an environmental disaster could have been responsible. The poisonous fumes could easily be from a nuclear disaster, such as a nuclear reactor being destroyed by the weather, just as what has recently happened in Japan. However, equally we could argue that the Ruin was the result of a World War or something like that.